An image sensor is a device which receives light from an optical image or one or more objects and converts the received light into electrical signals forming an image. Recently, with the development of the computer industry and communication industry, the demand for an image sensor having improved performance is increasing in various fields or applications, including, e.g., a digital camera, a camcorder, a portable device such as a personal communication system, a game machine, a security camera, a medical micro-camera and a robot.